Allow scrolling a page with selection after long-press on whitespace

ui/touch_selection/longpress_drag_selector.h

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_TOUCH_SELECTION_LONGPRESS_DRAG_SELECTOR_H_
 #define UI_TOUCH_SELECTION_LONGPRESS_DRAG_SELECTOR_H_
 
 #include "base/time/time.h"
 #include "ui/gfx/geometry/point_f.h"
 #include "ui/gfx/geometry/vector2d_f.h"
@@ skipping 22 matching lines @@
   ~LongPressDragSelector() override;
 
   // TouchSelectionDraggable implementation.
   bool WillHandleTouchEvent(const MotionEvent& event) override;
   bool IsActive() const override;
 
   // Called just prior to a longpress event being handled.
   void OnLongPressEvent(base::TimeTicks event_time,
                         const gfx::PointF& position);
 
+  // Called when a scroll is going to happen to cancel longpress-drag gesture.
+  // TODO(mohsen): This is a workaround to avoid starting longpress-drag gesture

[mfomitchev, 2016/01/04 17:07:55]

It seems like the root of the issue is that (without this fix) TSC calls
OnSelectionActivated() on LongPressDragSelector when a scroll happens. It does
so because it interprets the selection _update_ (resulting from a scroll) as a
_new_ selection - because it has response_pending_input_event_ set to
LONG_PRESS.

Can we please describe this in a bit more detail? Also, I suggest we the
description of the issue to TSC::OnScrollBeginEvent() (in the .cc file). We
should also mention that this happens when the long press does not result in a
selection.

[mohsen, 2016/01/04 18:45:59]

Done.

+  // if the longpress is happening on an empty area far from an existing
+  // selection. Get rid of this workaround when we have enough information about
+  // the cause of the selection (see https://crbug.com/571897).
+  void OnScrollBeginEvent();
+
   // Called when the active selection changes.
   void OnSelectionActivated();
   void OnSelectionDeactivated();
 
  private:
   enum SelectionState {
     INACTIVE,
     LONGPRESS_PENDING,
     SELECTION_PENDING,
     DRAG_PENDING,
@@ skipping 10 matching lines @@
   gfx::PointF touch_down_position_;
 
   gfx::Vector2dF longpress_drag_selection_offset_;
   gfx::PointF longpress_drag_start_anchor_;
   bool has_longpress_drag_start_anchor_;
 };
 
 }  // namespace ui
 
 #endif  // UI_TOUCH_SELECTION_LONGPRESS_DRAG_SELECTOR_H_